1. Field of the Invention.
The present invention relates to a power supply arrangement in a microwave oven comprising a magnetron driven by a Switch Mode Power Supply having a resonance circuit fed from the mains via a mains rectifier and comprising a transformer, which is connected to the magnetron via a voltage multiplier and delivers a driving voltage to the same and a controllable switch to be set and reset between closed and open conditions at a given switching frequency. The power delivered by the resonance circuit to the magnetron is dependent upon the switching frequency. A current transformer is included in a feedback circuit for sensing the current through the magnetron and the output signal of which is led to a control circuit for controlling the switching frequency by a comparison with a reference signal in order to regulate the switch frequency and thereby the power fed to the magnetron to a value determined by the reference signal.
The output power of a magnetron has a linear relationship to the anode current as the anode voltage can be regarded as constant. As a measure of the magnetron power it is therefore possible to use the anode current. Then a current sensing device, for example, a current transformer producing a signal corresponding to the DC-mean value of the anode current is required.
2. Description of Related Art.
A power supply arrangement according to the above is described in NL 7707605. The primary winding of the current transformer is included in the anode circuit of the magnetron. Accordingly, the anode current is directly measured by the current transformer. However, this involves a serious drawback due to the fact that the anode current has a very irregular waveform and contains strong disturbances, which will make the utilization of the feedback signal difficult and will require a filtering operation. Disturbances in the anode current may be caused by, for example, changes in the microwave impedance due to the character of the load or the position of the agitator.
It is to be noted that DE Offenlegungschrift 2 217 691 discloses a voltage multiplier in the output stage of a SMPS magnetron of the kind used in the power supply arrangement of the invention. However, there is no feedback signal from the voltage multiplier to regulate the switch frequency and thereby the power fed to the magnetron.
As a further example of prior arts, DE-OS 27 28 616, which corresponds to U.S. Pat. No. 4,096,559 (6/20/78), may be mentioned. The current flowing in the magnetron is sensed and used as a feedback coupling. It is not shown in detail in what way the current is sensed, but the use of a current transformer connected into a branch of a voltage multiplier must be excluded due to the simple fact that no voltage multiplier is shown or proposed.